A common type of spectrometer for inorganic analysis is an atomic emission spectrometer having an induction coupled plasma ("ICP") source of light radiation. The induction plasma generator utilizes an electrical induction coil to deliver high power at high frequency to excite a gas into a plasma. A nebulized sample material is injected into the plasma where it becomes disassociated into atoms which are excited in the plasma to emit radiation including spectral lines characteristic of the atomic elements in the sample. An example of such an induction plasma system is disclosed in U.S. Pat. No. 4,766,287 (Morrisroe et al).
Light radiation from the ICP is passed into a detector system that typically incorporates a monochromator or polychromator. A monochromator passes a selected wavelength of radiation to a detector, as disclosed in U.S. Pat. No. 4,326,802 (Smith et al). A polychromator disperses the radiation into a band or multiplicity of wavelengths that are detected. An example of a precision polychromator is an echelle system with crossed gratings to produce a two-dimensional display of spectral lines as disclosed in U.S. Pat. No. 4,820,048 (Barnard). The spectral lines are focused onto a detector formed from a two dimensional solid state charge transfer device which effects signals proportional to the intensity of the corresponding lines. A computer processes the signal information, corrects for background, applies calibration, and displays the results in the form of concentrations of atomic elements in the sample.
The ICP typically is mounted to the spectrometer so that radiation emitted radially from the ICP is directed into the detector system, as illustrated in the aforementioned U.S. Pat. No. 4,326,802. In the example disclosed therein, a mirror is receptive of the radial radiation, the mirror and associated lens components having an adjustable position longitudinally to optimize the location of atomic emissions.
The ICP also has been mounted longitudinally so as to direct radiation from along the central axis of the ICP into the detector system. Radial emission has been preferred for analytical accuracy, because the axial radiation has more self absorption along the longer longitudinal path in the plasma plume, and there are chemical interferences in the axial radiation from the fringe zone in the plasma where the sample material is not maximally atomized. However, the axial radiation has a longer emission path which provides greater sensitivity for low emission levels. Thus the longitudinal mounting for the ICP is often desirable when there are low levels of an atomic element in the sample being tested. Where flexibility has been desired for selecting either higher accuracy or higher sensitivity, it has been necessary to change the mounting of the ICP, which generally has not been a practical procedure on a repetitive basis, or to utilize two spectrometers which is costly.
Various systems have been used for optical switching. For example, the aforementioned U.S. Pat. No. 4,326,802 discloses the use of a mirror positionable by rotation to select between the radial radiation from the ICP and a calibration source of light. U.S. Pat. No. 4,622,468 (Stefanski et al) illustrates the use of beam splitters and shutters in a fluorescence detection system with a halogen lamp light source. A concave mirror is rotatable on an offset axis parallel to the central axis for switching as disclosed in U.S. Pat. No. 4,261,638 (Wagner). Other optical switches include rotating periscopes and Porro reflectors such as disclosed in U.S. Pat. No. 5,005,934 (Curtiss).
However, none of the foregoing references teaches selection of either radial or axial radiation from an ICP. Therefore, an object of the invention is to provide an improved atomic emission spectrometer having a capability for selecting either radial or axial radiation from an ICP. Another object is to provide an improved ICP atomic emission spectrometer having selectivity between maximum accuracy and maximum sensitivity. A further object is to provide such a spectrometer with speed and convenience in making the selection.